1. Field of the Invention
This invention relates to processes for recovery of heavy petroleum from subterranean formations and more particularly to such processes which employ a fluid medium to displace the petroleum.
2. Description of the Prior Art
In many oil-bearing subterranean formations the crude oil is under a pressure which forces it out of the formation to a production well from which it can be brought to the surface. However, in the case of heavy oil reservoirs, this recovery of crude oil, which is generally designated as the primary recovery phase, results in the recovery of only a very small part of the crude, if indeed of any at all. This is due to the fact that heavy oils flow poorly or not at all due to their high viscosity under the initial conditions in the reservoir. For this reason heat is supplied to such reservoirs, which decreases the viscosity of the crude, thus rendering it mobile.
Heat is usually supplied into the reservoir by the injection of hot water or steam into an injection well located at a suitable distance from the production well(s). In addition to heating the reservoir, the injected water or steam acts at the same time as a displacing agent for the crude oil which has become mobile as a result of the supply of heat. This does considerably increase the yield from a heavy oil reservoir, or makes it possible in the first place, but this yield is still far below the yields obtained in deposits of medium and light crude oils. One reason for this is that the viscosities of the displacing agents, hot water or steam, are much less than that of heavy oil, for which reason the heavy oil is not completely displaced. Moreover, when steam-drive is used, the existence of different densities causes the steam to concentrate in the upper part of the reservoir and to break through rapidly to the recovery well, so that it is not possible to build up a pressure gradient in the reservoir which is sufficient to displace the crude.
On the other hand, it is known that the degree of displacement in the recovery of crude oil by flooding can be considerably increased if small amounts of viscosity-increasing hydrophilic polymers are added to the flooding water. Suitable polymers are, for example, polysaccharides, hydroxyethylcelluloses and especially unhydrolyzed or only partially hydrolyzed polyacrylamides. However, since these chemicals are generally temperature-sensitive, they cannot be readily used for the recovery of heavy oil by hot water flooding.
Hence a need has continued to exist for a method of displacing heavy crude from underground formations which avoids the problems of the methods used hitherto.